Managing/Reducing noise in a night scene
Scenes with many light sources (like this one) tend to be noisy because by default cycles randomly picks one light source to sample for every ray.

In such scenes a decent improvement can be gained by sampling all lights at once for every sample, by using the branched path integrator:


Note that I rendered at 50% resolution

I also:


Enabled clamping, which does a bit of what combining multiple renders with different seeds does (at least when you use a format like png). Note that direct clamping can cause some rather visible dimming of light sources, so see if you can get away with just indirect clamping.
Turned down the number of bounces to gain some speed.
Turned off reflective caustics to mitigate fireflies cast by glossy surfaces.




Here are some other things which may help:


See if you can reduce the number of light sources. For example, the street lamps have a mesh light and a point lamp. Try using just one or the other. For example you might make the mesh light only visible to the camera, and let the point lamp do all the illuminating: 


Try and favor lamps over mesh lights where reasonable. Lamps can be sampled in a more optimized fashion. Don't feel like you have to replace every mesh light though, some places mesh lights are just there for a reason ;)
Enable multiple importance sampling on bright lamps/mesh lights or large surfaces which reflect a lot of light. I.e. relatively large contributors to the overall ambient illumination.
Disable multiple importance sampling on surfaces which contribute relatively little to the ambient illumination.
Make bump mapping only visible to the camera, to avoid slowing down sampling on indirect bounces where bump mapping makes very little visible difference.
See if you can get away with only using the environment texture for reflections:



The first frame shows the render with the environment map (visible in the preview), the second shows it without (world appears a solid color to the camera), and the third shows it with the environment map only affecting the glossy shader (solid color again).